Aspects of the invention relate to a turret assembly for a vessel, comprising a moonpool defined in the vessel and a turret structure rotatably mounted in said moonpool, wherein the turret structure comprises a turret table which by means of a main bearing is rotatably mounted in said moonpool and a hollow turret shaft defined by a surrounding shaft wall, which turret shaft has an upper shaft end connected to the turret table, a substantially cylindrical shaft part that extends downwardly from the turret table and a lower shaft end provided with lower means intended for cooperation with a lower bearing.
Such a turret assembly allows a vessel to weathervane around the turret structure which at its lower end (typically at the lower end of the vessel section, possibly near the elevation of the keel of the vessel) is provided with lower means (such as a chain table or for example a detachable buoy) for mooring purposes and/or for supporting risers and umbilicals for, for example, transfer of gas, fluids, power and signals. The main bearing defines the connection between the moonpool (vessel) and the turret structure (turret table) and its function is predominantly to transfer mooring, riser and inertia loads from the turret into the vessel while allowing weathervaning of the vessel. The turret table may support a turntable or process manifold, as is generally known for such a turret assembly.
The functions of the lower bearing are among others to limit the horizontal force on the main bearing and the bending moment in the shaft. The lower bearing generally and purposely transfers forces in a horizontal direction. The lower bearing may be a sliding bearing type with an initial gap (also known as “clearance” or “play”) of a few millimetres, amongst others to allow the vessel to deform (“sagging” and “hogging”) without “pinching” or without excessive “pinching” of the turret. The clearance can also occur over lifetime due to wear of the lower bearing components.
The clearance decouples the chain table from the vessel and therefore any horizontal loads on the chain table travel up the turret shaft to the main bearing. This causes bending moments in the turret shaft and additional loads on the main bearing, both of which could be substantial. These effects are largely limited by the closing of the lower bearing which occurs when the turret shaft has sufficiently bent so that the chain table closes the gap with the vessel. Further increase of the horizontal load will substantially be transferred from the chain table into the vessel trough the lower bearing.
There are a number of options for promoting the closure of the lower bearing and to reduce free motions within given play, such as for example decreasing the size of the lower bearing gap, adopting an alternative design for the lower bearing, pre-compressing the chain table against the vessel during the installation of the turret (for removing initial free play) and the use of structural elements with a smaller Young's modulus than steel (for example rubber).